Why scanned profile surfaces do not match individual hits

[[Wi...]]

Sirs:

I am measuring a linear profile that is based on a CAD file. When measuring the surface, the profile fails. Understandable. BUT when I take SPACE POINTS on the NOMINAL points that make up the profile, I see a big difference. I assumed the nominal points would be "measureable" with space points--to get a reasonable idea of what's going on. Question: would this difference lie in the fact that Calypso "makes" nominal points (for measuring) based on a calculated spline from the nominal points in the curve feature? That is, the space points might not exactly match the calculated spline nominal points for the scanned profile. Any explanation would be appreciated.

Thanks.

[[Ch...]]

You have mentioned CALYPSO. If this is not only a typo/mistake then please ask your question here: ZEISS #measuringhero Community - Index page. 

But let me answer the question for the ZEISS INSPECT software. If you observe differences between line profiles between the GD&T method and with clicked points than most properly you are not measure your points in the correct alignment as its established from the datum system.